Naturally occurring as well as adaptively developed Foxp3+CD4+ regulatory T cells (Treg) are central to the maintenance of immunological self-tolerance and immune homeostasis by suppressing aberrant or excessive immune responses. Treg specifically express the transcription factor Foxp3, which mediates the coordinate activation of genes such as CTLA-4 and GITR along with silencing of cytokines such as interleukin-2, IL-17 and interferon-?, that are normally expressed by effector T cells (Teff). The functional boundary between Treg and the Teff they are meant to suppress is not absolute. In fact, the flexibility between Treg and Teff may be an important component to both physiologic and pathophysiologic immune responses. Understanding this functional interchange requires an understanding of how immune effector genes are normally silenced in Treg. Despite progress in understanding mechanisms of Foxp3-dependent gene activation, the molecular mechanism of Foxp3-dependent gene repression has not been elucidated. Recently, we identified Eos, a zinc-finger transcription factor of the Ikaros family, as a critical mediator of Foxp3-dependent gene silencing in Treg. Eos interacts directly with Foxp3 and is necessary for gene silencing without affecting expression of Foxp3 activated genes. We further demonstrated that Eos and its corepressor C-terminal binding protein 1 (CtBP1) are necessary for histone modifications and ultimately promoter methylation involved in selective gene silencing in Treg. Knockdown of Eos in Treg abrogates their ability to suppress immune responses in vitro and in vivo. We hypothesize that Eos-CtBP-1 gene silencing is a highly regulated process in Treg that impacts on their function as suppressors vs. effectors of immunity. In the current proposal, we will further explore metabolic and microRNA dependent mechanisms of regulation of Eos-CtBP1 gene silencing in Treg as well as the in vivo consequences of ablation of this pathway for Treg development, homeostasis and modulation of adaptive immune responses. Specifically, we will 1) Elucidate the role of metabolic stimuli that affect NADH/NAD balance in Eos mediated gene silencing via the CtBP1 complex, 2) Study the regulation of Eos expression in Treg cells by microRNAs and 3) Study the consequences of Eos deletion to Treg cell homeostasis, differentiation and adaptive Treg cell development.